Leadframe for semiconductor packages

ABSTRACT

A leadframe for semiconductor packages. The leadframe includes a die pad, a side rail, a tie bar, and a plurality of leads. The side rail is around the die pad. The tie bar connects the die pad and the side rail. The leads extend from the side rail to close proximity to the die pad. Each lead has a corresponding lead relative to a predetermined center line. A predetermined pair of corresponding leads are substantial asymmetrical with each other in appearance relative to the predetermined center line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to leadframes for semiconductor packages and inparticular to leadframes for high frequency applications.

2. Description of the Related Art

Semiconductor dies are enclosed in plastic packages that provideprotection from hostile environments and enable electricalinterconnection between the semiconductor die and a printed circuitboard via a metal leadframe. The conventional leadframe typesemiconductor package has a central supported die pad for supportingsemiconductor die, a plurality of leads peripherally located therein, aplurality of bonding wires for electrically connecting the semiconductordie to the leads, and a mold compound, such as plastic, forencapsulating these components in a package structure.

In most semiconductor package configurations, a portion of the leadframeis internal to the package, (i.e., completely surrounded by the moldcompound). Portions of the leads of the leadframe typically extendexternally from the package body for electrically connecting the packageto the printed circuit board.

In the electronics industry, there is continued demand for developingsemiconductor dies which have increasing processing speeds and higherdegrees of integration. For a semiconductor package to accommodate theseenhanced semiconductor dies, the number of leads included in thesemiconductor package must be significantly increased. To avoid anundesirable increase in the size of the semiconductor packageattributable to the increased number of leads, a common practice is toreduce or narrow the spacing between the leads. However, a decreasedspacing between the leads increases the capacitance between the leads,and increases the level of self inductance and mutual inductance. Thisinductance adversely affects the quality of signals transmitted on theleads of the leadframe by increasing signal reflections; causing greaterimpedance mismatches.

Especially, in high frequency applications the semiconductor package hasthe greatest influence on total performance of the circuit, and one ofthe main causes of performance degradation is inductance of theinterconnections between chip and printed circuit board. Therefore, asthe operating frequency of these circuits increases, there is a need foreven lower impedance mismatches packages. As shown in FIG. 2A,conventionally, the lead route or lead distribution of the leadframe issubstantially symmetrical for desired productibility ormanufacturibility and lower process cost, but do negatively affect theimpedance match.

BRIEF SUMMARY OF THE INVENTION

The invention provides leadframe for semiconductor packages and a methodutilizing the same, providing flexible impedance match design, improvingthe electrical performance of the resulting electronic products.

The invention provides leadframe for semiconductor packages comprising adie pad, a side rail, a tie bar, and a plurality of leads. The side railis around the die pad. The tie bar connects the die pad and the siderail. The leads extend from the side rail to close proximity to the diepad. Every lead has a corresponding lead relative to a predeterminedcenter line. A predetermined pair of corresponding leads issubstantially asymmetrical with each other relative to the predeterminedcenter line.

Further scope of the applicability of the invention will become apparentfrom the detailed description given hereinafter. It should beunderstood, however, that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

A detailed description is given in the following embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood by reading the subsequentdetailed description and examples with references made to theaccompanying drawings, wherein:

FIGS. 1A through 1E illustrate top views of a preferred embodiment ofthe invention;

FIGS. 2A through 2B illustrate a conventional symmetrical leadframe;

FIGS. 3A and 3B illustrate a first experimental example of theinvention; and

FIGS. 4A and 4B illustrate a second experimental example of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best-contemplated mode of carryingout the invention. This description is made for the purpose ofillustrating the general principles of the invention and should not betaken in a limiting sense. The scope of the invention is best determinedby reference to the appended claims.

FIGS. 1A through 1E are top views of a preferred embodiment of theinvention. Referring to the FIGS. 1A through 1E, the leadframe comprisesa die pad 10, a side rail 30, tie bars 21 through 24, and a plurality ofleads. The side rail 30 is around the die pad 10. The tie bars 21through 24 connect the die pad 10 and the side rail 30. The leads extendfrom the side rail 30 close proximity to the die pad 10. In some cases,the side rail 30 is removed in a trimming or separation step of thesubsequent semiconductor packaging process.

The invention provides the capability to vary the impedance of anelectrical device. As examples, impedance can be controllably varied bychanging: the length of the leads; the pitch of the leads; the spacingbetween the leads; and/or the width of the leads. In consequence, inorder to respectively adjust the impedance of the leads, the inventionprovides an asymmetrical leadframe structure.

In FIGS. 1A through 1E, each lead has a corresponding lead relative to apredetermined center line at opposite location of the leadframe. In thisembodiment, an exemplary center line 50 is shown in FIGS. 1A through 1E.For example, the lead 241 corresponds to respective leads 141 a through141 e in respective FIGS. 1A through 1E, and the lead 245 corresponds tothe lead 145. In this embodiment, the leadframe comprises a pair ofcorresponding leads including the lead and the corresponding lead,substantially asymmetrical to each other. Specifically, thisasymmetrical design serves for impedance matching. For example, the lead241 is substantially asymmetrical with the respective leads 141 athrough 141 e in FIGS. 1A through 1E relative to the predeterminedcenter line 50. FIGS. 1A through 1E show various examples ofasymmetrical leads for the asymmetrical leadframe structure.

In some cases, the corresponding lead of a specific lead depends on theselected center line, such as the center line 50 of this embodiment. InFIG. 1A, for example, the lead 141 a corresponds to the lead 241relative to line 50. Also, the lead 141 a corresponds to the lead 146relative to a center line (not shown) passing through the space betweenthe leads 143 and 144. Further, the lead 141 a corresponds to the lead341 relative a center line (not shown) passing through and aligned withthe tie bars 21. In this embodiment, the center line 50 is utilized asthe exemplary center line in subsequent discussion.

Referring to FIGS. 1A through 1E, lead 141 a through 141 e areasymmetrical to lead 241 relative to center line 50, either the geometryof the lead or the route of the lead. Accordingly, the pair ofcorresponding asymmetrical leads related to the center line means thatthey are not identical in shape, dimension, or the relationship ofitself to other corresponding parts of the leadframe.

In FIG. 1A, the leads 141 a and 241 have different lengths, and thus,are considered to be asymmetrical. In consequence, comparing the pair ofcorresponding lead 141 a and lead 241, the varied lead length results invaried resistance of the lead 141 a. Thus, a desired impedance value canbe achieved by adjusting the lead length.

In FIG. 1B, the leads 141 b and 241 have substantially the same widths.However, space S₁ between lead 141 b and the adjacent lead, for examplelead 142, is larger than space S₂ between lead 241 and the correspondingadjacent lead, for example lead 242. Further, the pitch P₁ is alsolarger than the pitch P₂. Thus, the leads 141 b and 241 are consideredto be asymmetrical. In consequence, comparing the pair of correspondinglead 141 b and lead 241, varied space between the leads results invaried inductance between the leads. Thus, a desired impedance value canbe achieved by adjusting space between the leads.

In FIG. 1C, the leads 141 c and 142 c respectively have different widthsfrom the corresponding leads 241 and 242. Further, space S₁ between lead141 c and the adjacent lead, for example lead 142 c, is less than spaceS₂ between lead 241 and the corresponding adjacent lead, for examplelead 242. And thus, the leads 141 c and 241 are considered to beasymmetrical, and the leads 142 c and 242 are considered to beasymmetrical. In consequence, comparing the pair of corresponding lead141 c and lead 241, or lead 142 c and lead 242, varied lead widthresults in varied resistance of the lead. Thus, a desired impedancevalue can be achieved by adjusting the lead width.

In FIG. 1D, the pitch P₁ between the lead 141 d and the adjacent lead,such as lead 142 d, is larger than the pitch P₂ between the lead 241 andthe corresponding adjacent lead, such as lead 242. Thus, the leads 141 dand 241 are considered to be asymmetrical. In consequence, comparing thepair of corresponding lead 141 d and lead 241, varied lead pitch resultsin varied inductance between the leads. Thus, a desired impedance valuecan be achieved by adjusting the lead pitch.

In FIG. 1E, pitch P₁ between the lead 141 e and the adjacent lead, suchas lead 142, is less than the pitch P₂ between the lead 241 and thecorresponding adjacent lead, such as lead 242. Thus, the leads 141 e and241 are considered to be asymmetrical. In consequence, comparing thepair of corresponding lead 141 e and lead 241, the varied lead pitchresults in varied inductance between the leads. Thus, a desiredimpedance value can be achieved by adjusting the lead pitch.

Next, a conventional symmetrical leadframe is shown in FIGS. 2A through2B, and two experimental examples of the invention are respectivelyshown in FIGS. 3A, 3B and FIGS. 4A, 4B verifying the improvedperformance of the embodiment.

In FIG. 2A, a top view of a conventional semiconductor package is shown.The package comprises a leadframe, a semiconductor chip 2100 attached toa die pad 2010 of the leadframe, a plurality of bonding wires 2200electrically connecting the semiconductor chip 2100 and the leads of theleadframe, and an encapsulant (not shown) encapsulating thesemiconductor chip 2100, the leadframe, and the bonding wires 2200. Theleadframe comprises a die pad 2010, four tie bars 2021 through 2024 forsupporting die pad 2010, and a plurality of leads. The side rail wastrimmed during the packaging process. The conventional leadframe whichthe routes of the leads are substantially symmetrical.

In FIG. 2B, a magnified drawing of the exemplary leads 1145, 1146, 1148,and 1149 in FIG. 2A is shown. For an electronic signal with a frequencyof approximately 750 MHz, the differential impedance values of adifferential pair of the leads 1145 and 1146 is near 68 ohm. Similarly,the differential impedance value of a differential pair of the leads1148 and 1149 is near 68 ohm. And the single-ended impedance values ofthose leads 1145, 1146, 1148, and 1149 are near 50 ohm. In some case,however, the desired differential impedance values for some leads arerequired between 80 and 120 ohm, and preferably approximately 100 ohm.Or the desired single-ended impedance values for some leads are requiredbetween 40 and 60 ohm, and preferably approximately 50 ohm. Thus, theutilization of the conventional symmetrical leadframe cannot achieve thedesired impedance value.

In FIG. 3A, a top view of a semiconductor package of a firstexperimental example of the invention is shown. Compared to that shownin FIG. 2A, the lengths of the leads 1145, 1146, 1148, and 1149 arereduced by D, which is approximately 60 mils in this embodiment. Thus,the leadframe utilized in the package shown in FIGS. 3A can act asanother embodiment of the invention.

A magnified drawing of the shortened leads 1145, 1146, 1148, and 1149 isshown in FIG. 3B. For an electronic signal with a frequency ofapproximately 750 MHz, the differential impedance values of adifferential pair of the leads 1145 and 1146 is near 84 ohm, whichachieve the desired values. Similarly, the differential impedance valueof a differential pair of the leads 1148 and 1149 is near 84 ohm, whichachieves the desired values, too. And the single-ended impedance valuesthereof are near 58 ohm. It is appreciated that the package of the firstexperimental example utilizes the leadframe structure of the inventionto cause the impedance values of the predetermined leads fulfilling thedesired values for impedance match.

In FIG. 4A, a top view of a semiconductor package of a secondexperimental example of the invention is shown. Compared to that shownin FIG. 3A, spaces between the leads 1145 and 1146, and the spacebetween the leads 1148 and 1149 are broader. Thus, the leadframeutilized in the package shown in FIGS. 4A can act as another embodimentof the invention.

A magnified drawing of the leads 1145, 1146, 1148, and 1149 of FIG. 4Ais shown in FIG. 4B. For an electronic signal with a frequency ofapproximately 750 MHz, the differential impedance values of adifferential pair of the leads 1145 and 1146 is near 108 ohm, whichachieve the desired values. Similarly, the differential impedance valueof a differential pair of the leads 1148 and 1149 is near 108 ohm, whichachieves the desired values, too. And the single-ended impedance valuesthereof are near 62 ohm. It is appreciated that the package of thesecond experimental example utilizes the leadframe structure of theinvention to cause the impedance values of the predetermined leadsfulfilling the desired values for impedance match.

The efficacy of the inventive leadframes at developing asymmetrical leadroute or lead distribution provides effective impedance match for theresulting products.

While the invention has been described by way of example and in terms ofpreferred embodiment, it is to be understood that the invention is notlimited thereto. To the contrary, it is intended to cover variousmodifications and similar arrangements (as would be apparent to thoseskilled in the art). Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

1. A leadframe for semiconductor packages, comprising: a die pad; a siderail around the die pad; a tie bar connecting the die pad and the siderail; and a plurality of leads extending from the side rail to closeproximity to the die pad; wherein every lead has a corresponding leadrelative to a predetermined center line; and a predetermined pair ofcorresponding leads are substantial asymmetrical with each otherrelative to the predetermined center line.
 2. The leadframe as claimedin claim 1, wherein the leads of the predetermined pair comprisesubstantially different lead lengths.
 3. The leadframe as claimed inclaim 1, wherein the leads of the predetermined pair comprisesubstantially different lead widths.
 4. The leadframe as claimed inclaim 1, wherein the leads of the predetermined pair comprisesubstantially asymmetrical extending traces.
 5. The leadframe as claimedin claim 1, wherein the leads of the predetermined asymmetrical pair arefor impedance matching.